The occlusion effect is generally described as the sensation of increased loudness (sound pressure level), especially in the low frequencies, that a person experiences to self-generated sounds (vocalization, chewing, swallowing, walking, and the like), when the ears are covered (occluded). Note that this resonance amplification can occur in tubes that have a sealed volume and have acoustic leakage into the volume. The occlusion effect has been identified as a major obstacle to successful hearing aid use and shallow (within the first ½ of the channel) inserted earpieces. The theories of why the occlusion effect forms and what it is are numerous and diverse and to date no single explanation has been totally accepted.
FIG. 3 illustrates typical occlusion effect levels as a function of frequency for various in-ear devices.
There are several theories of the occlusion effect. They include outflow theory (Mach, 1863): occlusion of the ear canal results in an increase in middle ear impedance, and hence to a decrease in energy lost from the inner ear via the ossiculaer chain. Resonance theory (Huizing, 1923): increased perception of sound is brought about by the walls of this artificially closed cavity acting as resonators. Masking theory (Pohlman, 1930; Hallpike, 1930): occlusion of the ear canal eliminates masking influence of ambient noise. Inertial/osseotympanic theory (von Bekesy, 1932): the occlusion effect results from sound pressure increase in the auditory canal with occlusion. Inertia of the mandible to the skull sets up pressure variations in EAM. Impedance theory (Huizing, 1960): occlusion alters the impedance of the column of air in the canal (increasing it), resulting in improved coupling of the air in the canal to the middle ear.
FIG. 4 illustrates several occlusion effect studies and their values at various frequencies for earphones, while FIG. 5 illustrates several occlusion effect studies for earmolds. Roughly the occlusion effect is in the range of 13-25 dB between 250-500 Hz. Roughly from Killion, Wilber, and Gudmundsen (1988) a shallow insertion has an occlusion effect of about 13 to 21 dB, while a deep insertion has an occlusion effect of about 20 dB for a tapered tip, and about −9 to 4 dB for a bony contact ear inserted device. Related art solutions involve acoustic vents between the sealed region (now unsealed) and the outside environment of about 3 mm in diameter, however venting has limitations as well, for example ringing. Another solution is deep insertion with contact in the bony section of the ear canal.
Thus for shallowly inserted systems (e.g., <½ the ear canal length), the occlusion effect can be an issue (e.g., >5 dB).